1. Field of the Invention
The present invention relates to fourdrinier papermaking by which a continuous web is formed from a jet of fibrous slurry flowing through a slice opening in a headbox. More particularly, the present invention relates to a gauge or tool for measuring the three interdependent dimensions of a papermachine headbox slice opening.
2. Description of the Prior Art
In the continuous, fourdrinier method of manufacturing paper, a slurry of aqueously suspended fiber jets from the elongated slice opening of a headbox onto a traveling drainage screen. The fiber constituency of the slurry is retained on the screen surface to form an accumulated mat or web while the aqueous vehicle drains through the screen pores.
One of the more highly sought quality characteristics of paper made by this method is a uniform cross-directional basis weight: i.e. a uniform weight of dry fiber per unit area across the width of web so formed. A significant basis weight profile control parameter is the headbox slice profile. This profile is essentially defined by three, interrelated dimensions: (1) the slice opening, (2) the slice lip projection and (3) the apron length.
The slice apron is that structural component of a headbox serving as the lower support element for the slurry as it flows across the headbox slice opening. The slice lip is that structural component of a headbox having a very thin longitudinal lower edge which defines or delineates between the headbox interior and exterior. A headbox slice beam is the front wall of the headbox to which the slice lip is structurally secured albeit accommodation is given to adjustment of the slice lip in the vertical plane by means of numerous, uniformly spaced slice adjusting rods or screws.
From these components, the slice opening is that distance between the lower edge of the slice lip to the upper surface plane of the slice apron. The slice lip projection is that distance between the lower edge of the slice beam and the lower edge of the slice lip. The slice apron length is that distance between the terminal end of the slice apron and the vertical plane of the slice lip.
Interdependence of these slice dimensions arises from the complex structure of the headbox slice beam and automatic control over the beam and lip profiles. There are two dominant stress sources upon the beam. Thermal stress due to temperature differences between the slurry and the ambient atmosphere surrounding the headbox may warp or bow the beam in a large, shallow arc from deckle to deckle. The resulting geometric consequence of these stresses on the slice profile normally is not planar-parallel with the slice apron plane, i.e. the bow causes a slice lip having a uniform lip projection to yield a slice opening in the slice center different from that at the deckle edges. Another source of stress on the headbox beam having slice opening profile consequences is the hydraulic head of the slurry therewithin. If uncorrected, a basis weight gradient in the web may result.
Correction of such a beam bow usually takes the form of automatic slice lip adjustment by which the electronically scanned basis weight profile is translated to motorized rotation of the slice lip adjusting rods. If properly calibrated, this slice lip adjustment will level the slice opening relative to the apron notwithstanding the beam bow. Simultaneously, profiled differentials will arise in the lip projection and apron length dimensions.
To a certain degree, such profiled differentials in the slice lip and apron length dimensions are acceptable. When the acceptable limits are exceeded, however, other control forces must be brought to bear on the slice beam itself. For these reasons, it is essential that the slice lip be correctly calibrated relative to the slice beam and slice apron. Such correct calibration requires that the three slice lip dimensions hereto described begin from uniform or at least known settings when the headbox is empty and at a stable ambient temperature. This requires a very careful, consistent, and extremely precise manual measurement of the dimensions at approximately 30 locations across the slice width: many of which must be made from an awkward position.
It is, therefore, an object of the present invention to provide a light and easily manipulated gauge that is adapted for measuring all three of the critical slice opening dimensions.